Compensating for voice effort curves by adjusting the anode potential on open grid tubes

At your risk. The softer the voice the more bass and treble (bandwidth)[2] it contains. If we reproduce a soft voice at higher intensity it will sound like bass heavy like mud on a flat frequency responce system as the Fletcher Munson curves come into aditional play too. In order for such a recording to sound correct ie flat frequency response from producer's brain to listener's brain, bass or treble must be cut ie amplifier bandwidth reduced accordingly. This sometimes explain why cheap mics may do an excellent vocal job at certain applications. The softer the voice and the higher the reproduced intencity the less the bandwidth needed. A way to peform such adjustments with least components is to use an open grid electron tube preamplifier stage and adjust just the plate (anode) voltage. At low anode potential less anode current takes place, the higher the anode internal resistance and the higher the bass cutoff frequency by its (ra) interaction with the primary inductance of the electron tube stage output transformer. Conversly for a loud singing voice performace we may increase the Vb supply and increase bandwidth and headroom. A low plate potential can create a very low noise front end amplifier which is convenient for a soft singing peformance. Further reading: Pleiades V6 1H4 amplifier schematic. Sound Picture Recording and Reproducing Characteristics - D. P. Lowe, K. F. Morgan - Journal of the Society of Motion Picture Engineers Operating Features of the Audion - E. H. Armstrong